It can provide both over-voltage and under-voltage protection at the same time, there are three types of model depending on the input voltage. For the range of protection, it can be used for 3-phase or single-phase depending on the model. 110V Type: 110~150V for over voltage, 80~120V for under voltage, 220V Type: 220~300V for over voltage, 160~240V for under voltage, 440V Type: 380~500V for over voltage, 300~440V for under voltage. The 3-phase line voltage is displayed at an interval of 5 seconds; the operation time for over voltage and under voltage can be set separately.